How to preserve shape keys with decimate modifier?
For this you must re-create all shape keys for each LOD-version of your mesh. You can try to do this. First making LOD-version by applying Decimate modifier to copy of your high-poly mesh with all shape keys deleted. 

Then, sequentially turn on each shape key on your original high-poly mesh and apply Shrinkwrap modifier to previously made LOD-version as a shape key targeting that high-poly mesh with shape key turned on. 

That is, add Shrinkwrap modifier to low-poly ('decimated') mesh. In your high-poly mesh turn on particular shape key by set it value to 1. Select your high-poly mesh as a target in Shrinkwrap modifier. Click 'Apply as Shape Key' on Shrinkwrap modifier. If all went good you would had particular shape key transferred from high-poly to low-poly model.